Re: [PATCH] blk-throttle: fix divide-by-zero on legacy iops limit of 0

[David Laight <david.laight.linux@gmail.com>]

On Tue, 14 Jul 2026 18:35:52 +0800
Tao Cui <cui.tao@linux.dev> wrote:

> From: Tao Cui <cuitao@kylinos.cn>
> 
> Writing a multiple of 2^32 (e.g. 4294967296) to a legacy cgroup v1
> throttle iops file (blkio.throttle.{read,write}_iops_device) silently
> truncates to 0: tg_set_conf() stores the sscanf-parsed u64 value into
> an unsigned int field with no clamping. The cgroup v2 path,
> tg_set_limit(), already clamps the same kind of value with
> min_t(u64, val, UINT_MAX), but the legacy path never did. Note that
> the "!v -> U64_MAX" mapping only catches an explicit zero and does not
> catch a value that truncates to zero.
> 
> With iops stored as 0, tg_update_has_rules() sets has_rules_iops[] and
> the next IO reaches tg_within_iops_limit(), which computes
> 
>     jiffy_wait = max(jiffy_wait, HZ / iops_limit + 1);
> 
> triggering a divide-by-zero oops.
> 
> Fix it in two places:
> 
>   * tg_set_conf(): clamp the value to UINT_MAX, consistent with
>     tg_set_limit(). This closes the truncation root cause (and the
>     general silent truncation for any value above UINT_MAX).
> 
>   * tg_dispatch_iops_time(): treat iops_limit == 0 as unlimited so the
>     divide in tg_within_iops_limit() is never reached, defending
>     against any future path that could produce a zero limit.
> 
> Signed-off-by: Tao Cui <cuitao@kylinos.cn>
> ---
>  block/blk-throttle.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/block/blk-throttle.c b/block/blk-throttle.c
> index ffc3b70065d4..3f3c1374f4b2 100644
> --- a/block/blk-throttle.c
> +++ b/block/blk-throttle.c
> @@ -883,7 +883,12 @@ static unsigned long tg_dispatch_iops_time(struct throtl_grp *tg, struct bio *bi
>  	u32 iops_limit = tg_iops_limit(tg, rw);
>  	unsigned long iops_wait;
>  
> -	if (iops_limit == UINT_MAX || tg->flags & THROTL_TG_CANCELING)
> +	/*
> +	 * iops_limit == 0 is not a valid limit. Treat it as unlimited so we
> +	 * never reach the HZ / iops_limit divide in tg_within_iops_limit().
> +	 */
> +	if (iops_limit == UINT_MAX || iops_limit == 0 ||
> +	    tg->flags & THROTL_TG_CANCELING)
>  		return 0;
>  
>  	tg_update_slice(tg, rw);
> @@ -1386,7 +1391,8 @@ static ssize_t tg_set_conf(struct kernfs_open_file *of,
>  	if (is_u64)
>  		*(u64 *)((void *)tg + of_cft(of)->private) = v;
>  	else
> -		*(unsigned int *)((void *)tg + of_cft(of)->private) = v;
> +		*(unsigned int *)((void *)tg + of_cft(of)->private) =
> +			min_t(u64, v, UINT_MAX);

The LHS casts look horrid - there has to be a nicer way to do that.

And you don't need min_t() a plain min() will be fine.

	David



>  
>  	tg_conf_updated(tg, false);
>  	ret = 0;